Various monitoring devices with cameras, infrared sensors, or the like have been developed to monitor and detect people entering and leaving buildings, sites, and facilities or walking on the street as subjects, thereby identifying suspicious individuals.
On the other hand, in addition to identity verification using IC cards, various user authentication methods for access are in practical use based on what is called biometric information unique to the authenticatee. Such biometric information includes a fingerprint pattern, an iris pattern, blood vessel patterns in the hand, arm, and back of the eye, and facial characteristics.
Of these biometric authentication methods, an authentication method based on facial characteristics (hereinafter, face authentication) is implemented in various monitoring devices because of their simplicity in structure and authentication. Such monitoring devices include room entry control systems installed inside or at the entrances to buildings, buildings, premises, and facilities.
Face authentication is performed as follows. The authenticatee (hereinafter, the subject) is photographed including his/her face to generate a facial image, which is used to generate authentication information. The authentication information is compared with registered authentication information (registered authentication information) and if they coincide with each other, the subject is authenticated as a registered one.
In the case of monitoring devices, people moving through area under surveillance are detected and tracked based on the difference between the background image in the area and an input image at the time of being photographed (see, for example, Patent Document 1 below).
In the case of monitoring devices for room entry control, people entering and leaving rooms or buildings are tracked to check their flow lines so as to determine whether the people are already registered or not (see, for example, Patent Document 2 below).
These conventional devices, however, have a drawback that the time required to verify people in the image area increases with increasing number of people. As another drawback, when the monitoring devices track and verify people moving from the places of the devices, the time required to obtain the verification results greatly varies depending on the photographic environment such as lighting conditions at each photographing time, facial direction, and the camera-to-subject distance.
Under such circumstances, when performing real-time tracking and verifying of moving subjects, the monitoring devices need to complete photographing and verification processes within the time in which the subjects can be tracked so as to prevent insufficient monitoring. This requires the monitoring devices to have high processing capability. As a result, these monitoring devices are undesirably large in circuit size to achieve parallel processing and high-speed processing, and hence expensive.    Patent Document 1: Japanese Patent Unexamined Publication No. H08-205131    Patent Document 2: Japanese Patent Unexamined Publication No. 2004-185484